Phone no longer powers on

Phone had slow pulsing blue light with nothing on the screen. Would not power on, or any combination of buttons to reset/power up. Brought to local cell phone fixer and was unable to determine or fix, now essentially dead. They initially stated maybe power plug which is connected to multiple buttons. Can this be the mainboard or daughterboard? Or digitizer? Screen is not crack and for the most part the phone looks fine. Just trying to see of there's a way of troubleshooting the problem or salvage this expensive device.

Thanks.